Passive radiotherapy intensity modulator for electrons

ABSTRACT

Typically, electron beam radiation therapy aims at delivering a uniform dose to a target volume containing cancer cells. Electron sources typically impinge a spatially uniform flux across the beam onto the patient; however, irregular patient and bolus surfaces, the latter encountered in bolus electron conformal therapy (ECT), scatter electrons unevenly creating non-homogeneous dose distributions in the target. However, spatially-modulated beam intensities can restore target dose homogeneity, as well as enable utilization of other advanced ECT methods. Unfortunately, present methods, which have attempted to spatially-modulate beam intensities, have been either impractical or ineffective. Here, a novel, passive method has been developed to spatially-modulate electron beam intensities by taking advantage of multiple Coulomb scattering. The method utilizes Island Blocks or Island Apertures, strategically located in ‘transparent’ or ‘opaque’ substrates, respectively, which are placed in the beam&#39;s path. This method spatially-modulates electron flux across the beam with insignificant loss of electron beam energy. Thus, delivering a uniform, highly conformal dose distribution to the target volume is possible. Further, the method is inexpensive and can be easily incorporated into existing electron therapy machines.

CROSS REFERENCE

The present application claims benefit of U.S. Provisional PatentApplication No. 62,533,923, filed 18 Jul. 2017, which is herebyincorporated by reference herein in its entirety, including but notlimited to those portions that specifically appear herein.

FIELD OF THE INVENTION

Electron beam therapy (“EBT”) is one form of radiation treatment ofpatients, particularly those patients with certain forms of cancer wherethe target cells are found in tissues within 6 cm of the surface.

Typically, the energy of an electron beam used in EBT is between about 6and 20 million electron volt (“MeV”).

Other radiation sources, including X-rays (photon beams) and chargedheavy particle beams (typically proton beams), also are used to treatcancer.

When treating a patient, a distribution of dose, typically uniform, isprescribed to a specified volume of tissue, often called the planningtarget volume (PTV), which typically is the volume containing thetargeted cancer cells plus some margin.

As a radiation beam penetrates the patient's body (or other target), theradiation beam deposits energy. The deposition of energy per unit massis defined as radiation dose.

The physical mechanisms by which these beams deposit dose depend on themodality of the radiation, i.e. electrons, protons, or photons. Further,scattering of radiation beams affects the deposition of dose. As anX-ray or photon beam penetrates a body, its dose deposition decreasesapproximately exponentially. A proton beam penetrates a fixed depth(defined as the range) in the body, depositing a sharp peak of dose justbefore stopping, that depth depending on the beam energy. Hence, aproton beam requires energy modulation in order to spread its peak doseover the PTV size in the direction of the beam. An electron beam alsopenetrates a fixed depth in the body, depositing dose approximatelyuniformly for approximately 67% of its range, then decreasingcontinuously over the next few cm.

It is desirable to control the spatial distribution (modulation) of theincident intensity or flux of a beam. As used herein, the termsintensity and flux are used to mean the number of charged particles orphotons per unit area at a position in a radiation beam, which impacts atarget.

When treating a patient, it is normally desirable for there to be a nearuniform dose within the PTV, while at the same time minimizing dose tohealthy tissues. Treatment of deep seated PTVs requires either highenergy X-ray or proton beams, each having multiple beams incident fromdifferent directions. Often the best treatments require these beams tobe intensity modulated, called intensity modulated X-ray therapy (IMXT)or intensity-modulated proton therapy (IMPT). Hence, many X-raytreatments require beams from multiple directions, each beam havingintensity modulation. Similarly, many proton treatments require beamsfrom multiple directions, each beam having energy (or range) andintensity modulation.

Superficial PTVs can be treated with electrons, and often the besttreatment is delivered by electron conformal therapy (ECT), requiringenergy and intensity modulation. Bolus electron conformal therapy (BECT)normally uses a single beam of sufficient energy to reach the deepestpart of the PTV; then energy (range) in the patient is modulated by avariable thickness of approximately unit density material (e.g. wax)called bolus. However, electrons scattering from the non-uniform surfaceof the bolus or of the patient's body cause the dose in the PTV to benon-uniform, which can be removed by intensity modulation (IM) referredto as IM-BECT.

A second ECT method, called Segmented Field ECT, utilizes multiple,abutted fields from the same direction, but of differing energy. Beingof different energies, two abutting fields have differing penumbralwidths, resulting in volumes of increased and decreased dose (referredto as hot spots and cold spots), which can be removed by using intensitymodulation to broaden the sharper of the two penumbras to match theother. A third method, coined modulated electron radiation therapy(MERT) utilizes modulated beams of differing beam energy and directions.Hence, all three methods of ECT require intensity modulation.

X-rays and protons, at the energies used for treating cancer patients,exhibit little scattering in the air between the beam source and thepatient. Thus, intensity modulation for X-ray beams has been done withlittle difficulty using dynamic multileaf collimators (MLCs) located far(60-70 cm) from the patient. MLCs form multiple beams, each of differingshapes and intensities, so when summed form any specified intensitydistribution. However, electron beams show significant scattering in theintervening air, so that X-ray MLCs cannot be used for electronintensity modulation. The air scatter prohibits the required intensitymodulation at standard treatment distance, and although reducedtreatment distances partially resolve that problem (issues withscattering from the side of X-ray leaves remain), they place thetreatment table and patient at unsafe, impractical positions. Hence,X-ray MLCs have not emerged as a solution for electron intensitymodulation.

Passive methods for achieving X-ray intensity modulation utilizemetallic blocks of variable thickness, which control intensity versusposition by controlling X-ray attenuation versus position. Thesedevices, called X-ray compensators, are highly effective, but cannot beused for electrons because electrons are not attenuated.

Protons, because of their large mass, do not show significant scatter inair; hence, proton beam intensity modulation has been achieved bymagnetic beam scanning, which varies the proton intensity as orthogonalbending magnets scan the beam over a PTV. Beam scanning is highlyeffective for proton beams, but cannot be used for electrons because oftheir significant amount of scatter by intervening air and the endwindow of the accelerator.

The invention disclosed herein describes a novel method for passiveradiotherapy intensity modulation for electrons (referred to sometimesas ‘PRIME’).

BACKGROUND AND DESCRIPTION OF PRIOR ART

Three types of electron conformal therapy (“ECT”) either require orcould benefit from intensity modulated (IM) electron beams:segmented-field ECT, bolus ECT, and modulated electron radiation therapy(“MERT”); however, no reliable method for IM of ECT is available.

Intensity modulation of an X-ray beam has been accomplished usingmultileaf collimators (“MLCs”), which are collimators with an assemblageof “leafs” which selectively block sections of an X-ray beam. Attemptsthat have been made to deliver electron intensity modulation usingphoton MLCs have proven unsuccessful. When X-ray MLCs are used withelectron beams, scattering in the large air gap between the MLCs and thePTV nullifies any useful modulation of the electron beam. Thus, photonMLCs have been ineffective in IM for ECT.

Electron multileaf collimators (“eMLC”) have been made and arecommercially available, but because these devices are heavy, cumbersome,not permanently attached to the electron accelerator, and expensive, fewif any eMLCs are used. Further, eMLCs must be placed very close to thepatient, for example within 10-15 cm, which makes it even more difficultto use an eMLC. In comparison, photon MLCs typically are placed about60-70 cm from the patient and are attached to the electron accelerator.

Proton beam IM, best achieved by modulating the beam as it ismagnetically scanned across the PTV, is possible due to proton multipleCoulomb scattering (MCS) in air being of such small magnitude (few mm).Such is not the case for electrons. Even replacing the intervening airwith a gas such as helium, which causes relatively low MCS, isinsufficient due to MCS from the accelerator end window and ion chamber.Furthermore, scanning electron beams have remained off the market sincethe 1990s as a result of deadly accidents that occurred with scannedelectron beam machines. These approaches are impractical for routinetreating of patients, and thus to date, these approaches remaincommercially unavailable.

Although not actually IM, when EBT is used to treat an eye retina, asingle island is placed over the patient's eye. Such an island blocksthe electron beam from directly impinging on the underlying lens, whichis at a shallow depth (0.3 cm), thereby reducing the possibility ofblindness or cataracts formation. However, electron MCS allows theunderlying retina, which is at a deeper depth (3.0 cm), to receive about70% of the prescribed dose.

Also not actually IM, on some occasions, EBT utilizes two or moreelectron beams having different energies (also known as Segmented FieldECT). However, due to differing beam energies or beam misalignment,unwanted hot spots and/or cold spots can result in the treatment volume.To reduce these unwanted results, saw-toothed collimator edges have beenused to match the electron penumbra (e.g. 80%-20% dose edge) forabutting beams of differing energies.

The invention disclosed herein is a novel method for passive electronintensity modulation that comprises inserting a plurality of IslandBlocks (sometimes referred to as “Islands”) and/or Island Apertures(sometimes referred to as “Apertures”) in the path of an electron beam.This method provides control of electron beam intensity modulationwithin the treatment field and in the penumbra. The pattern of IslandBlocks and/or Island Apertures may be optimized for any particular EBTtreatment. This method is low-cost and can be easily incorporated intomost existing clinical settings.

SUMMARY OF THE INVENTION

A novel method for Passive Radiotherapy Intensity Modulation forElectrons (“PRIME”) is disclosed herein. This novel method utilizes asimple insert placed in or near the aperture of an electron beamcollimator, which generates an arbitrary intensity pattern as a resultof electron beam scattering by air above the collimator and subsequentdrifting from the collimator to the PTV. This method is inexpensive andcan be used with existing electron beam sources with minimal changes.

PRIME delivers a therapeutic electron beam intensity, in a planeperpendicular to the central electron beam axis, which may be modulatedin a controlled manner across the treatment field.

PRIME comprises a method of allowing an electron beam, generated in amanner well known to those in the field, to pass through an intensitymodulator comprising a collection of small area Island Blocks placedinside or adjacent to a collimating insert and/or a collection of smallIsland Apertures placed throughout the collimating insert. The intensitymodulator impinges the specified intensity modulated electron beam onthe target. Island Blocks, as used herein, are defined as an array ofsolid, electron blocking posts or rods implanted in an electrontransparent substrate (e.g. thin, low-density, machineable foam slabs).Island Apertures, as used herein, are holes or electron transparentpathways arrayed on an electron collimating substrate (e.g. lead,Cerrobend, or copper). A collimating insert, as used herein, is a deviceplaced in the path of an electron beam for the purpose of shaping thecross section of the beam to that best suited to irradiate the patientPTV.

The Island Blocks may vary in height, shape, cross sectional area, andspacing between Islands. The Island Apertures may vary in height, shape,diameter, and spacing between Apertures. The Island Blocks must beconstructed of medically non-hazardous electron-blocking material, suchas tungsten, titanium, lead, lead alloy, copper, steel, nickel, or othersuitable heavy metals or their alloys. The Island Blocks are placed intoa thin, low-density, substrate (e.g. low-density machineable foam) thatis placed inside or near the electron field-defining metal (e.g.Cerrobend or copper) collimating insert that defines the beam shape. TheIsland Apertures are holes in the collimating insert, which vary inshape, diameter, and spacing between Apertures. Under normalcircumstances, the central axes of the Island Blocks and IslandApertures will closely coincide with rays diverging from the virtualelectron source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays a beam's eye view (looking down the central axis of abeam from the virtual radiation source) of a prototype electron beamintensity modulator containing only Island Blocks.

FIG. 2 displays an Island Block model showing a side view of the Islandsin an electron-transparent substrate.

FIG. 3 displays an Island Aperture model showing a side view of theApertures in an electron-blocking substrate.

FIG. 4 displays a depiction of the electron intensity when an electronbeam passes through air without an electron beam intensity modulator.

FIG. 5 displays a depiction of the electron intensity when an electronbeam passes through an electron beam intensity modulator containingIsland Blocks.

FIG. 6 displays a relative intensity profile 2 cm deep in water under aline through the middle of the set of Island Blocks in the upper half ofthe collimating insert (larger Blocks on left to smaller Blocks onright) for the prototype intensity modulator shown in FIG. 1.

FIG. 7 displays the contour map of isointensity lines created by anelectron beam after passing through the patient passive intensitymodulator shown in FIG. 8 and designed using the method disclosedherein.

FIG. 8 displays a schematic of a beam's eye view of a patient intensitymodulator, which includes Island Blocks of varying diameters located ona hexagonal grid and embedded in an electron-transparent substrate thatfits within the collimating aperture of the collimating insert definingthe electron beam.

DETAILED DESCRIPTION OF THE INVENTION

The method comprises selecting the areas of the Island Blocks or IslandApertures, selecting the shape of the Island Blocks or Island Apertures,selecting the separation between Island Blocks or Island Apertures,precisely locating the Island Blocks in an electron-transparentsubstrate, or for Island Apertures, locating the hole or apertureprecisely through an electron-blocking substrate, and placing the IslandBlocks or Island Apertures in the path of an electron beam. When theelectron beam passes through the Island Blocks and/or the IslandApertures, a desired intensity-modulated, non-homogenous flux ofelectrons is delivered to a target.

FIG. 1 displays a prototype electron beam intensity modulator [1] usingIsland Blocks, which is located inside the aperture of the collimatinginsert [10]. The figure shows an electron transparent substrate [3] intowhich a plurality of Islands [5] are imbedded. Typically, the Islands[5] are sized and spaced in a manner to optimize the spatialdistribution of electron beam intensity at a target, for example, apatient. This prototype is a test pattern in which Island Blocks areequally spaced and have systematic changes in diameter.

FIG. 2 provides a side view of a portion of a cut through an electronbeam intensity modulator [1] using Island Blocks, showing the electrontransparent substrate [3] and the Island Blocks [5] opaque to theelectrons (stops and absorbs electrons).

FIG. 3 provides a side view of a portion of a cut through an electronbeam intensity modulator [2] using Island Apertures, where the substrate[7] is opaque to electrons and the Apertures [9] allow passage of theelectrons.

Whereas in general EBT is hindered by electron multiple Coulombscattering (MCS), this method takes advantage of MCS by air, as anelectron beam travels from its source to the target. As an electron beamencounters the array of Islands Blocks, some electrons pass through thesubstrate and some electrons are blocked (stopped and absorbed). Asshown in FIG. 4, electrons at each point in the beam [15], having beenscattered by air upstream of the electron collimator [10], spread asthey drift through air [11] from the electron collimator [10] to thetarget plane. There the individual electron beamlet distributions [12]recombine, producing a relatively uniform manner [13] at a predictabledepth.

FIG. 5 illustrates the effect of placing the Island Blocks [5] in thepath of an impinging electron beam [15]. Electrons at each point in thebeam still spread as they drift through air [11] due to their beingscattered by air upstream of the electron collimator [10], but some ofthe beam [15] is stopped by the Island Blocks [5]. Thus, when theelectron beam recombines further downstream at the target, the resultingintensity [17] is non-uniform (modulated). Electrons that are notblocked still scatter and recombine at a predictable distance below theIsland Blocks [5] in a predictable manner. The same mechanism appliesfor Island Apertures [9], except more of the electron beam is blockedand intensities will be lower.

Using this novel method, some electrons are blocked and thus never reacha target. Therefore, a distinct pattern of electron flux reaches thetarget. FIG. 6 displays an electron intensity profile of a 16 MeVelectron beam at a depth of 2 cm in water (surface 5 cm downstream ofintensity modulator) along a line passing through the center line [4] inFIG. 1 of a prototype passive radiotherapy intensity modulator forelectrons (PRIME), where the relative intensity of the intensitymodulated electron beam versus position is plotted at the target. Theintensity of the resulting electron beam is selectively reduced ormodulated at selected locations as the beam reaches a target. Thedistance below the PRIME containing Island Blocks [5] (or IslandApertures [9]) at which the scattered electron beamlets recombine, andthe desired intensity is achieved, may be controlled by the size (area)and separation of the Islands [5] (or Apertures [9]).

As an electron beam encounters a pattern of Island Blocks (or IslandApertures), modulation of the intensity of the electron beam occurs,thereby causing a non-uniform, predetermined intensity distribution toimpinge the target tissue, which results in a nearly uniform dose to thepatient PTV. FIG. 7 illustrates the dose modulation for a patient usingthe electron beam modulator displayed in FIG. 8. FIG. 7 is an isocontourmap of a modulated intensity impinging on a patient. The topographicallines, 90, 92, 94, 96, and 98, represent the percentage of the full dosereaching that area in the absence of the intensity modulator.

Island Apertures [9] may be used instead of Island Blocks [5] when it isdesirable for a higher fraction (≥50%) of the electron beam to beblocked. Island Apertures [9] may be thought of as the inverse of IslandBlocks [5]. FIG. 5 illustrates Island Blocks [5] stopping electrons,with the low-density substrate [3] allowing electrons to pass through.Inversely, FIG. 3 illustrates electrons passing through Island Apertures[9], with the collimating substrate [7] stopping the electrons.

The Island Blocks [5] comprise columns with essentially flat tops withshapes selected from circles, rectangles, squares, hexagons, or otherpolygons. The surface area of the sides of the columns should beminimized as compared to the surface area of the tops of the columns tominimize the fraction of electrons scattering into or from the columnwalls; hence, the most preferable cross sectional shape of the columnsis circular. The columns may be parallel to the central axis of the beam(perpendicular to the substrate) or at other angles, most preferably atan angle aligned with the divergence of the beam (projected from thevirtual source of the electron beam), again the latter to minimize thefraction of electrons scattering into or from the column walls.

The height or thickness of the Island Blocks (columns) [5] must besufficient to completely block (stop) an electron beam at the energiestypically used in EBT. The actual thickness depends on the material usedfor construction. For example, Island Blocks constructed of tungstenmust be at least 0.6 cm thick for beams up to 20 MeV.

The diameters of cylindrical Island Blocks [5] are typically betweenabout 0.1 cm and 1.0 cm, which for non-circular cross sectionscorrespond to areas between about 0.008 cm² and 0.8 cm².

The substrate [3] into which the Islands [5] were imbedded comprises alow density material, such as a low density polymer.

For Island Apertures [9], the size of the Apertures [9], the thicknessof the substrate [7] and the distance between Apertures [9] defines themodulated flux in the same manner as described for Island Blocks [5].

It is important to note that the novel method, PRIME, described herein,spatially modulates the intensity of an electron beam with insignificantchange to the beam's mean energy or energy distribution.

For hexagonal packing of cylindrical blocks, the range of Island Block[5] diameters and separation parameters (“d, r”) may be varied todetermine the pattern of Intensity Reduction Factors (“IRF”), where

$\begin{matrix}{{{{IRF}\left( {d,r} \right)} = \left\lbrack {1 - {\left( \frac{\pi}{2\sqrt{3}} \right)\left( \frac{d}{r} \right)^{2}}} \right\rbrack},} & (1)\end{matrix}$where d is the diameter of the cross section of the Island Blocks [5]and r is the distance between Island Blocks [5]. For non-circular crosssections, 2A

$\begin{matrix}{{{{IRF}\left( {d,r} \right)} = \left\lbrack {1 - \left( \frac{2A}{\sqrt{3}r^{2}} \right)} \right\rbrack},} & (2)\end{matrix}$where A is the cross sectional area of the Island Blocks.

Circular Island Blocks packed in a hexagonal grid, for example, may beused for intensity modulated ECT, for which IRFs in the range of 0.70 to1.00 are expected.

The resulting intensity of an electron beam is defined as follows:I _(desired) =I _(o)*IRF,  (3)where I_(o) is the initial, unmodulated intensity of the electron beam.

Equation 4, which is a rearrangement of equation 1, may be used todetermine the preferred diameter of cylindrical Island Blocks [5] ateach point within a hexagonal grid, as shown below,

$\begin{matrix}{{d\left( {r,{IRF}} \right)} = {{r\left\lbrack {\frac{2\sqrt{3}}{\pi}\left( {1 - {IRF}} \right)} \right\rbrack}^{\frac{1}{2}}.}} & (4)\end{matrix}$

Each Island Block [5] may impact multiple locally desired intensities.Thus, the exact diameter and location of the Island Blocks [5] may beoptimized to obtain the desired intensity distribution incident on thetarget.

Whereas Island Blocks [5] are likely more effective for 0.50≤IRF≤0.99,Island Apertures are likely more effective for 0.01≤IRF≤0.50. Thisminimizes the undesirable effect of electrons scattering into or out ofthe sides of the Island Blocks and Island Apertures.

For hexagonal packing of cylindrical Apertures, the range of IslandAperture [9] diameters and separation parameters (“d, r”) may be variedto determine the pattern of Intensity Reduction Factors (“IRF”), where

$\begin{matrix}{{{{IRF}\left( {d,r} \right)} = {\left( \frac{\pi}{2\sqrt{3}} \right)\left( \frac{d}{r} \right)^{2}}},} & (5)\end{matrix}$where d is the diameter of the cross section of the Island Aperture [9]and r is the distance between Island Apertures [9]. For non-circularcross sections,

$\begin{matrix}{{{{IRF}\left( {A,r} \right)} = \frac{2A}{\sqrt{3}r^{2}}},} & (6)\end{matrix}$where A is the cross sectional area of the Island Apertures [9].

Circular Island Apertures [9] packed in a hexagonal grid, for example,may be useful for modulated electron radiation therapy (MERT), for whichIRFs in the range of 0.01 to 1.00 are expected.

The resulting intensity of an electron beam is defined as follows:I _(desired) =I _(o)*IRF,  (7)where I_(o) is the initial, unmodulated intensity of the electron beam.

Equation 8, which is a rearrangement of equation 5, may be used todetermine the preferred diameter of cylindrical Island Apertures [9] ateach point within a hexagonal grid, as shown below,

$\begin{matrix}{{d\left( {r,{IRF}} \right)} = {{r\left\lbrack {\frac{2\sqrt{3}}{\pi}{IRF}} \right\rbrack}^{\frac{1}{2}}.}} & (8)\end{matrix}$

Each Island Aperture [9] may impact multiple locally desiredintensities. Thus, the exact diameter and location of the IslandApertures [9] may be optimized to obtain the desired intensitydistribution incident on the target.

Island Blocks [5] and Island Apertures [9] can be used together whenthere is a wide range of intensity modulation, e.g. 0.01≤IRF≤0.99, as ispossibly the case for the central region of IMET beams and the edges ofsegmented ECT beams.

Preferably, the Intensity Reduction Factor (IRF) for bolus ECT should bebetween about 0.7 and 1.0 when using Island Blocks [5]. There aremultiple solutions of (d, r) for equation 3 for Islands [5] of circularcross-section packed in a hexagonal grid that provide an IRF of between0.70 and 1.00. As the values of (d,r) increases, the number of Islandsneeded decreases. However, if the values of (d, r) are too large, therewill be insufficient scatter beneath the Island Blocks [5] to create thedesired, reduced intensity distribution.

Example 1

As an example, FIG. 1 illustrates a collection of circular Island Blocks[5] of varying diameter located on a hexagonal grid (0.58 cm spacing)inside the aperture of a custom electron collimating insert. This is aprototype insert where an approximately square beam (field) ishalf-modulated. The diameters of the Island Blocks [5] comprising theintensity modulator vary from about 0.15 cm (IRF 0.94) in the right mostregion to about 0.4 cm (IRF 0.54) at the left. The Island Blocks [5]used in this example were made from tungsten and were 0.6 cm in length.

Example 2

The relative intensity profile for the prototype modulator described inexample one was measured 2 cm deep in a water tank with the distancefrom the electron source to the water surface at 100 cm SSD. FIG. 6illustrates the relative intensity profile (from left to right) throughthe center of the region containing Island Blocks [4] in FIG. 1.

Example 3

FIG. 8 depicts a modulator designed from an actual patient treated withelectron beam therapy. Here the diameter and placement of the IslandBlocks [5] were designed to deliver a more uniform dose to the patient'sPTV. The resulting intensity pattern incident on the patient is plottedusing isointensity contours in FIG. 7.

The invention claimed is:
 1. A method for passively modulating electronbeam intensity spatially for an electron beam used in electron beamtherapy comprising, a. generating the electron beam with an energybetween 6 and 20 million electron volts; b. directing the electron beamthrough a beam defining aperture of an electron beam collimator and anintensity modulating insert, which is located near it and near a patientplanning target volume; and impinging the electron beam, after it passesthrough the intensity modulating insert and the beam defining apertureof the electron beam collimator, onto the planning target volume of apatient, Wherein the intensity modulating insert comprises a pluralityof electron Island Blocks imbedded in an essentially electrontransparent substrate, wherein the electron Island Blocks are sized andspaced so that parts of the electron beam are blocked, while theremainder of the electron beam passes through the electron transparentsubstrate without loss of significant energy thereby producing themodulated electron beam with a two-dimensional, non-homogenous spatialdistribution of electron beam intensity, without significantly changingthe energy of said electron beam, and is patient specific.
 2. The methodas in claim 1 wherein the electron Island Blocks may be columns ofcircular, square, rectangular, hexagonal, or other polygonal crosssections.
 3. The method as in claim 1 wherein the electron Island Blocksare columns constructed of a dense metal of such thickness that itcompletely blocks electrons that impinge on said electron Island Blocks,wherein the dense metal is selected from the group consisting oftungsten, lead, copper, iron, and a heavy metal alloy.
 4. The method asin claim 3 wherein the electron Island Blocks are constructed oftungsten.
 5. The method as in claim 3 wherein the electron Island Blocksare constructed of a medically inert materials.
 6. The method as inclaim 1 wherein the electron Island Blocks have a cross-section witharea equaling that of a circle between 0.05 cm and 1.0 cm in diameter.7. The method as in claim 1 wherein the electron Island Blocks' centersfall on a hexagonal, rectangular, or other grid where grid locations areseparated by distances between 0.1 cm and 5.0 cm.
 8. A method forpassively modulating electron beam intensity spatially for an electronbeam used in electron beam therapy comprising, a. generating theelectron beam with an energy between 6 and 20 million electron volts;directing the electron beam through a beam defining aperture of anelectron beam collimator and an intensity modulating insert which islocated near the electron beam collimator and near a patient panningtarget volume; impinging the electron beam, after it passes through theintensity modulating insert and the beam defining aperture of theelectron beam collimator, onto the patient planning target volume,Wherein the intensity modulating insert comprises a plurality of IslandApertures arrayed in an electron blocking substrate, wherein the IslandApertures are sized and spaced so that parts of the electron beam passthrough the Island Apertures, while the remainder of the electron beamis totally blocked by the electron blocking substrate thereby modulatingthe electron beam energy without loss of significant energy therebyproducing the modulated electron beam with a two-dimensional,non-homogenous spatial distribution of electron beam intensity, withoutsignificantly changing the energy of said electron beam, and is patientspecific.
 9. The method as in claim 8 wherein the Island Apertures maybe circular, square, rectangular, hexagonal, or other polygonal crosssections.
 10. The method as in claim 8 wherein the electron blockingsubstrate is constructed of a dense metal that completely blockselectrons that impinge on the electron blocking substrate, wherein thedense metal is selected from the group consisting of tungsten, lead,copper, iron, and a heavy metal alloy.
 11. The method as in claim 10wherein the electron blocking substrate is constructed of medicallyinert materials.
 12. The method as in claim 8 wherein the IslandApertures have a cross-section with areas equaling that of a circlebetween 0.05 cm and 1.0 cm in diameter.
 13. The method as in claim 8wherein the Island Apertures' centers fall on a hexagonal, rectangular,or other grid where grid locations that are separated by distancesbetween 0.1 cm and 5.0 cm.
 14. A method for passively modulatingelectron beam intensity spatially for an electron beam used in electronbeam therapy comprising, a. generating the electron beam with an energybetween 6 and 20 million electron volts; b. directing the electron beamthrough a beam defining aperture of an electron beam collimator and anintensity modulating insert which is located near the electron beamcollimator and near a patient planning target volume; and impinging theelectron beam, after it passes through the intensity modulating insertand the beam defining aperture of the electron beam collimator, onto thepatient planning target volume, Wherein the intensity modulating insertcomprises a plurality of electron Island Blocks imbedded in anessentially electron transparent substrate in combination with aplurality of Island Apertures arrayed in an electron blocking substrate,wherein the electron Island Blocks and the Island Apertures are sizedand spaced so that parts of the electron beam are completely blocked,while the remainder of the electron beam passes through the intensitymodulating insert without loss of significant energy thereby producingthe modulated electron beam with a two-dimensional, non-homogenousspatial distribution of electron beam intensity, without significantlychanging the energy of said electron beam, and is patient specific. 15.The method as in claim 14 wherein the electron Island Blocks and IslandApertures may be the same shape or different shapes selected from thegroup consisting of circular, square, rectangular, hexagonal, and otherpolygonal cross sections.
 16. The method as in claim 14 wherein theelectron Island Blocks and the electron blocking substrate areconstructed of a dense metal that completely blocks electrons, whereinthe dense metal is selected from the group consisting of tungsten, lead,copper, iron, and a heavy metal alloy, wherein the dense metal used forthe electron Island Blocks may be the same as or different from thedense metal used for the electron blocking substrate.
 17. The method asin claim 16 wherein the electron Island Blocks are constructed oftungsten and the electron blocking substrate is constructed of copper.18. The method as in claim 16 wherein the electron Island Blocks and theelectron blocking substrate are constructed of medically inertmaterials.
 19. The method as in claim 14 wherein the electron IslandBlocks and the Island Apertures have cross-sections with areas equalingthat of circles between 0.05 cm and 1.0 cm in diameter.
 20. The methodas in claim 14 wherein the electron Island Blocks' centers fall onhexagonal, rectangular, or other grids where the grid locations areseparated by between 0.1 cm and 5.0 cm and the Island Apertures' centersfall on hexagonal, rectangular, or other grids where grid locations areseparated by between 0.1 cm and 5.0 cm.
 21. The method as in claim 1wherein the intensity modulating insert is located approximately 10 cmabove the planning target volume of the patient.
 22. The method as inclaim 8 wherein the intensity modulating insert is located approximately10 cm above the planning target volume of the patient.
 23. The method asin claim 14 wherein the intensity modulating insert is locatedapproximately 10 cm above the planning target volume of the patient.